The distribution of Met5-enkephalin-Arg6-Phe7 (Met-Enk-Arg-Phe)-like immunoreactivity in the rat gastrointestinal tract was studied by immunohistochemical techniques. Antiserum against a Met-Enk-Arg-Phe-throglobulin conjugate was raised in rabbits and was found to be specific for synthetic Met-Enk-Arg-Phe. Met-Enk-Arg-Phe-like immunoreactivity was found in neuronal structures in all parts of the rat gastrointestinal tract. Immunostained somata were primarily located in myenteric plexus; immunostained processes were mostly present in myenteric plexus and circular muscle layer. This distribution pattern is similar to that of the three other opioid polypeptides, Met5-enkephalin, Leu5-enkephalin and Met5-enkephalin-Arg6-Gly7-Leu8.